This invention relates to digitizer tablet systems and, more particularly, in a pen cursor for use in performing writing motions on the surface of a tablet, to a sensor for mounting in a tip portion of the pen cursor to develop a signal output indicating longitudinal force on a writing tip of the pen cursor comprising, a tip member extending from the tip portion of the pen cursor and mounted for longitudinal movement, the tip member having a plunger portion extending therefrom within the pen cursor; a resistive member disposed within the pen cursor behind the plunger portion; a conductive member disposed within the pen cursor adjacent the resistive member behind the plunger portion; compressing means for the plunger portion to force the resistive member and the conductive member together in electrical contact over a contacting area which is directly proportional to the amount of longitudinal pressure on the plunger portion; and, connection means for connecting a voltage across the contacting area whereby resistance as a function of longitudinal pressure on the plunger portion can be measured.
In a digitizing tablet system, a cursor is moved over the surface of a tablet to :input positional information related to an X,Y coordinate system associated with the tablet to a computer connected to the tablet. To allow the user to indicate when positional information is to be taken, and the like, the cursor typically has one or more manually-operable buttons associated with it. Recently, for use with so-called pen-driven computing systems where a digitizing tablet associated with a display is the primary input device for the computer, cordless digitizers having no physical connection between the cursor and the tablet have become the configuration of choice because of their added convenience to the user.
The elements of a typical cordless digitizer system are shown in simplified form in FIG. 1. The system 10 comprises a tablet 12 having a connecting cable 14 (or optical link) to the computer (not shown). There is also a cursor 16 having one or more buttons 18 thereon. The cursor 16 is typically about the size of an average fountain pen. In implementations according to techniques employed by the assignee of this application, the cursor 16 is powered by a small lithium battery of the type employed in hearing aids and the like. Accordingly, it can be appreciated that low power consumption as well as reliability are both important aspects of the manner of operation. The cursor 16 contains a coil which is driven by a pulse stream for positional sensing purposes. The coil emits a corresponding stream of AC magnetic pulses which are sensed by scanned sensing grids in the tablet. The signals developed in the sensing grids are then employed to determine the position of the cursor 1.6 on the surface of the tablet 12 according to techniques which are well known to those skilled in the art and which form no part of the present invention.
For handwriting analysis of operator inputs, and the like, as desired in the so-called pen-driven computing systems which are presently achieving increased attention and popularity, data about the angle of the cursor 16 and the pressure on the cursor 16 against the tablet 12 may be developed and transmitting to the tablet 12 for further transmission to the computer for use in the analysis process. A typical prior art approach to sensing the pressure of the cursor 16 against the tablet 12 so as to develop a signal reflecting that pressure for transmission to the computer is shown in FIG. 2. The cursor 16 has a longitudinally movable tip 20 which is biased to an extended position by a spring 22. As pressure is applied to the tip 20 the tip 20 slides into the body of the cursor 16 against the biasing force of the spring 22. The greater the pressure, the further the tip 20 slides into the body of the cursor 1.6. The tip 20 carries a light-blocking strip 24 on its inner end which moves in combination with the tip 20. There is a light emitting diode 26 and a photodiode 28 positioned on opposite sides of the path of the light-blocking strip 24. With the tip 20 fully extended (i.e. with no pressure on the tip 20), light from the light emitting diode 26 strikes the photodiode 28 fully and a maximum signal is output from the photodiode 28. As pressure is applied to the tip 20 and the tip 20 moves into the body of the cursor 1.6, the light-blocking strip 24 moves between the light emitting diode 26 and the photodiode 28 blocking a portion of the light striking the photodiode 28 and reducing the signal output therefrom. The light continues to be blocked in greater amounts until the photodiode 28 is completely blocked at the maximum pressure level which is sensible.
While the above-described apparatus of FIG. 2 does develop a signal which reflects the pressure on the tip 20, the longitudinal sliding movement required to develop that signal is unnatural during a writing action with the cursor as users are used to writing with pencils, pens, and ball-point pens which do not move longitudinally at their tips as a function of the pressure being applied. Additionally, the apparatus of FIG. 2 occupies necessary space in the body of the cursor 16 which can better be used for other purposes and has a high current consumption which is particularly undesirable in a cordless cursor where the total power available is a small lithium battery as mentioned above. Also, the apparatus of FIG. 2 has a high cost of manufacture and it is not easily adjustable for different applications. For example, the sensitivity is fixed by the amount of movement required to cover the photodiode 28 in amounts which will produce a detectable signal change and the spring constant of the spring 22.
As an alternate possibility and approach to the apparatus of FIG. 2, one could use a force-sensing resistor connected to the tip 20 such as those sold commercially by the Interlink corporation. While such a force-sensing resistor is preferable to the apparatus of FIG. 2, it also .has many of the same problems in that it is not easily adjustable for different applications.
A related approach is disclosed in U.S. Pat. No. 4,318,096 to Thornberg et al. in which the force-sensing element comprises the opposite (back) end of the shaft (instead of the tip or front end). No plunger element separate from the shaft and from the force-sensing element is provided. Therefore, the contact area between the small back end of the shaft and the force-sensing element is what varies with the applied force. The disadvantage is that the area of the back end of the shaft is very small.
Wherefore, it is an object of this application to provide a sensor for sensing longitudinal pressure on the tip of a digitizer pen cursor and developing an output signal reflecting the instantaneous pressure wherein there is virtually no detectable longitudinal movement of the pen tip.
It is another object of this application to provide a sensor for sensing longitudinal pressure on the tip of a digitizer pen cursor and developing an output signal reflecting the instantaneous pressure wherein characteristics of the sensor such as sensitivity are easily adjustable for different applications. It is still another object of this application to provide a sensor for sensing longitudinal pressure on the tip of a digitizer pen cursor and developing an output signal reflecting: the instantaneous pressure which is easy and practical to build in commercial quantities and which resists malfunctioning from misalignment during assembly.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.